Electronic converters are widely used in energy generation applications, such as in wind generators for example. Desired requirements of a converter include obtaining an output current with the best Total Harmonic Distortion (THD) level possible and obtaining a high voltage to minimize conduction losses.
Today, most manufacturers of electronic conversion stages are trying to increase their power by increasing the number of voltage levels in the output voltage. Not only does this increase the handled power but it also increases the quality of the delivered current by largely reducing the harmonic content.
The two preceding objectives can be achieved by means of using multilevel converters. These converters are capable of working with different voltage levels, to try to obtain an output current with the best THD possible, i.e., with the best wave quality possible. They are also capable of increasing the working voltage, which is a desirable characteristic for obtaining fewer losses.
A number of multilevel conversion topologies are known in the state of the art, such as that described in EP0555432A1, EP1051799B1, EP1673849A1, EP1815586A1, EP1287609A2 and in “Generalized Multilevel Inverter Topology with Self Voltage Balancing” by F. Z. Peng, IEEE Transactions on Industry Applications, Vol. 37, pp. 611-618, March/April 2001. Nevertheless, most of these topologies involve an excessively complicated and expensive electromechanical design.
For example, the converter described in “Generalized Multilevel Inverter Topology with Self Voltage Balancing” provides the previously specified characteristics for a multilevel converter, but it requires a large number of semiconductors and capacitors, which considerably increases converter size and cost.
Another multilevel converter providing the aforementioned characteristics is that described in EP0555432A1, which consists of n cells, made up of one capacitor and two semiconductors, series-connected one after the other. The increase in the available voltage levels is done by means of adding or taking away the capacitor voltages. However, this converter has the drawback of being large-sized and expensive due to the capacitors, which largely complicates the electromechanical design. The need for this large size limits the number of levels that can be reached by this converter because there comes a time when the required volume makes it no longer viable.
EP1287609A2 proposes a converter that allows reducing capacitor volume. Like in the converter disclosed in EP0555432A1 described above, the converter proposed in EP1287609A2 consists of cells series-connected, but in this case, each cell consists of two capacitors and three pairs of semiconductors. This enables achieving three voltage levels with each cell and, in the case of series-connecting n cells, 2*n+1 levels in total. The problem with cells of this type is that two pairs of semiconductors are arranged in series, which complicates control over distributing voltage among them.
EP1051799B1 proposes a multilevel converter called Active Neutral Point Clamped (ANPC) converter, consisting of a Neutral Point Clamped (NPC) type converter in which the level or clamp diodes are replaced with controlled semiconductors. On the other hand, if more than three output voltage levels are desired, it proposes intercalating capacitors in the output stage. The main problem with the converter described in EP1051799B1 is that to obtain a converter of n levels it is necessary to series-connect (n−1)/2 controlled semiconductors, complicating the distribution of voltages among said semiconductors.
EP1673849A1 attempts to solve this problem, disclosing a multilevel converter formed by series-connecting several switching units, each of which is made up of two controlled semiconductors arranged in series through a capacitor.
A problem shared by all the mentioned converters is the overvoltages occurring in the semiconductors in switching. These overvoltages are caused by stray inductances and in practice they reduce the current capacity and the service life of the semiconductors. The circuit described in EP1815586A1, using additional voltage limiting circuits and semiconductors, tries to mitigate this phenomenon, but these additional elements increase cost and size and make the converter more complex.
Therefore the technical problem is to be able to convert energy by obtaining an output voltage and current which minimize conduction losses without increasing the size, complexity or cost of the converter.